This is a list of different citrus in order of how much cold they can handle:

kumquat > mandarin > orange > grapefruit > pomelo

I might write:

orange > lemon > lime

I don't want to put them with grapefruit because there are different grapefruit varieties spanning the range from being as cold-hardy as orange to being as cold-sensitive as lime. Although kieffer limes are about as cold-hardy as orange (probably since they are believed to have some papeda in their ancestry).

Minneola tangelos are probably somewhere between mandarin and orange. Clementines and tangerines span the range between mandarin and orange.

Not only are pomelos the most vulnerable to cold, they also need a lot of heat to ripen. Despite being a pomelo-grapefruit hybrid, Oroblanco is probably one of the cold-hardier grapefruits, being close to orange in cold-hardiness. (Due to its comparatively smaller fruit size and sweetness it is also as easy to ripen as common grapefruits)

Cold hardy citrus varieties are considered to be able to grow well in zone 9. Some of the less common unusually cold hardy citrus can grow in zone 8, though usually they do better in zone 8b than 8a.Extremely cold hardy citrus hybrids may be able to survive in zone 7 unprotected, although they grow much better with a little protection during the winter. The ones that could survive in zone 7 unprotected are not going to be very edible. Possibly you may be able to grow fairly acceptable tasting citrus in zone 7b if you create a good microclimate.No citrus is going to be able to survive outside without a significant amount of protection in zone 6.

Most any citrus grows well in zone 10, although some pomelo varieties (which you don't find these varieties in the U.S. anyway) do not do so well over the winter. Even 'Chandler' experiences some leaf yellowing caused by the winter. After an uncommonly cold winter in the colder part of zone 10, a pomelo tree may not do well for the first half of that season, its growth will be cut back. Pomelos generally cannot be grown in zone 9, although Oroblanco (a Duncan grapefruit x pomelo hybrid) can grow fine in zone 9b

Texas Ruby Red grapefruit can be grown ok in zone 8b. Most American grapefruit varieties prefer zone 9 (or higher), and need some heat to ripen their fruits. (The first grapefruit varieties that were grown sometimes died off in zone 9 after particularly cold winters) Valencia is a great orange variety but needs heat to ripen its fruits to full flavor and sweetness, so it does not taste so good when grown in other parts of the country that don't have hot clear sky Spring and Summers.

Kumquats are a common one that can be grown in zone 8, and there are a few rare Japanese/Chinese citrus varieties that both taste good and do well in zones 9a and 8b. Satsuma mandarins are a fairly common (and great tasting) citrus variety that can handle 8b, a fact many people are not aware of. It's pretty unusual for such a common citrus variety to have so much cold hardiness.

I wonder if it is from your own experience? The hardiness of citrus hybrids is a very complex thing, it depends on the vegetative state of plants when exposed to the cold, its duration , absence or presence of sun light, humidity, rootstock etc. Citruses are reacting to these factors strikingly differentially, your description of their cold tolerance is rather misleading.From my experience classical white colored grapefruits are more hardy than oranges. Thomasville is less hardy than citrumelos and many citranges. Ichang papeda is breaking dormancy very rapidly and can be killed by sudden frost when Yuzu is unaffected.I have some pomelo seedlings that survived when similiar age grapefruits were completely destroyed.

What is published on the internet is not necessarily accurate and somewhat based on urban legends. My Arctic Frosts Satsumas which are supposed to tolerate 10oF are no more cold tolerant than my other Satsumas.

In my experience there is a loose relationship between vigorous growing citrus under moderate conditions and cold hardiness. For example, my Trifoliata seedlings are outgrowing the other citrus varieties with Rio Red Grapefruit also very vigorous. They have a better ability to bounce back and recover. A thicker, stouter tree will likely have more cold tolerance during freezing events. Of course this does not hold true for some types.

There needs to be a formal field comparison of cold hardiness to know for sure.

Gotta say, making a list of cold hardiness in citrus seems very difficult. I personally wondered, since citrus are influenced so much from enviromental variables, how do you even compare the hardiness? To get a reliable list, people should grow a BUNCH of different citrus, in rows, very close each other, on same rootstocks (ideally a dormant one, like the poncirus, so you are sure they are in same physiological stage when winter comes) under the same fertilization program, with plants of the same age. I doubt that something similar has been done, except maybe in research center, but still, research centers on citrus usually are in place so warm that some species aren't tested to the true extent of their cold hardiness. The fact that i live in a place that has a proper fall, helps a lot with acclimation, and i think this may explain some discrepancies i see from here to the US. Grapefruits, at least some of them, seem to be as cold hardy as orange if not more. I think a lot of people mix "heat seeking" with "frost tender". Grapefruit may need a lot of heat to make decent fruit but they don't seem bothered by frost. The only problem it's that you get acidic fruits.Kumquat are really hardy compared to other fruit bearing citrus but fruits can be damaged from cold. Lemons and oranges are on par. I see very sparse oranges there here, as much sparse as lemons tree. Clementines are more hardy thank people give them credit to. I had one this winter that went to -7°C (my nights in winter last for 14 hours) with just frost cloth and it didn't even flinched. It is grafted on bitter orange. Ironically i had a tahiti lime planted very close to that plant, own roots. Some twig dieback but the plant didn't even die to the ground.

Well, this year I learned that cold hardiness is largely based on micro climate, not just considering how warm you can keep your tree in winter ( south side of house, etc, but also how cold can you keep your tree during those weeks of warm weather we get here in the southeast US in January and February. I have a high tunnel that is opened/ closed with doors on both ends that I've been protecting my Ichang and poncyrus hybrids with for the past three winters with water barrels next to each tree. I'm on top of a mountain, south facing slope, with high tunnel running along slope horizontally west to east. High cold winds in winter. The doors don't seal very tight. The west side gets the highest coldest wind. The trees inside all did great last year with cold January and February and an easy transition into spring with no roller coaster temps. I keep doors open to vent tunnel to keep trees dormant when temps above freezing. This past January and February however had a lot of highs in 70's and even with doors open, it got too hot in tunnel and trees started pushing new growth in March when we got a night of 14 degrees and the next night 16 degrees. My low this winter was 7 degrees in January and trees were fine then. My Dunstan citrumelos that were protected somewhat from winds by other forest and orchard trees in the open yardwent through entire winter without dropping leaves. My citradia on south side of house with a water barrel on the west side of tree died down to the level height of the barrel. This citradia is also three feet away from blacktop driveway and had broke dormancy when 14 degrees hit. One big Dunstan out in open without a barrel lost half of its height due to getting full winds in the face. The trees in tunnel were more severely damaged on the east end of tunnel and the east side of each tree; the cold side of the trees is now growing better than the warmer east side of the trees. The order of the tunnel trees from west to east are as follows: Dunstan ( full green leaf with no damage and no water barrel ) , Ichang lemon ( now growing on west side of plant with few live shoots on east side and lost 3/4 height, no water barrel), pink seedling grapefruit from grocery fruit ( dead, water Barrel), Thomasville ( lost only top foot of growth from 6 ft tree but is green branches that still have not started to push growth yet, barrel), Ichang lemon ( water barrel, lost upper 5ft of 6 ft tall tree, starting to push growth from base trunk), Nansho daidai (barrel, lost 2 ft of 5ft tree and half the overall width of tree, now pushing growth vigorously at base and some half way up tree/branches, citradia ( lost upper 5 feet of seven foot high vigorous and wide tree, barrel, some sprouts from base and one sprout on branch 1 ft up from ground), rusk citrange ( barrel, lost upper two feet of growth and half of width of a 7 ft tall tree, pushing growth all over tree now), mortan citrange ( east end of tunnel by door, barrel, lost upper three feet of 7 ft tall tree, starting to push growth from base and a few sprouts on branches half way up tree. ) point: Dunstan is hardiest of all, protected or not. Ichang on colder west end of tunnel near door is doing better than the Ichang that was in warmer center of tunnel even without water barrel protection, and all fared better than citradia ,which is supposed to be the cold hardiest of them all, because citradia is in the center east end up hill end of the tunnel, was growing vigorously into fall, heat rises, etc. I should have taken tunnel down in February when highs in 70s , but it's too hard to put up again since plastic is stapled to heavy wooden framework to withstand high winds up here. All citrus need to stay dormant unti no more freezing temps in spring, and if breaks dormancy, needs to be kept from freezing, or you will get big damage regardless of variety.

From some sources, I've read that (or at least seen indication of) Ichang papeda is only very slightly cold-hardier than yuzu. In other words, Ichang papeda may be slightly less cold-hardy than commonly thought (at least under some conditions it seems) and yuzu can be more cold-hardy than commonly thought. These two are very closely related I think, closer than people realize (I think the story might be a bit more complicated than yuzu being a simple one-time hybrid cross, but that's my perspective).

The information I've come across has commonly indicated that yuzu is a little bit more cold-hardy than Changsha, but I don't really know if that's the case. They could be about equal, or Changsha might be ever slightly more cold-hardy in some situations. We have rough values for the temperatures these citruses can survive but it's hard to ascribe more precise values that are completely accurate to within 5 degrees F. I have never seen an experiment where several trees of each of these were compared with each other right around the critical cold temperature and where one type survived and one did not. Or I mean where one group clearly outperformed another group, with multiple trees from each group.

It almost seems like Ichang papeda may be able to survive lower temperature in absolute terms, but Changsha (and to a slightly lesser extent yuzu) has a better ability to bounce back after some cold damage and shows more resiliency. It also seems like yuzu has better ability to tolerate late freezes and fluctuating temperatures than trifoliate, even though trifoliate definitely can survive much lower temperatures in absolute terms.

Also wanted to mention here that Changsha mandarin shows the trait of being very drought tolerant as well.

I don't know, if I had to guess I'd guess yuzu is probably about 3 or 3.5 degrees (F) more cold hardy than Changsha. But if you're thinking about breeding cold hardy citrus, it might be easier to breed something edible from Changsha though, because Dr. Brown's two yuzu-clementine crosses didn't turn out that good, had some bitterness and kerosene smell; whereas the Changsha-Satsuma cross called "Orange Frost" has very good fruit quality. Yuzu might theoretically be more optimal to start off with, but it would probably take several more successive generations to breed out the undesirable attributes. Or it just might be that yuzu would be more appropriate if you were trying to breed something like a lemon.

I have never seen an experiment where several trees of each of these were compared with each other right around the critical cold temperature and where one type survived and one did not. Or I mean where one group clearly outperformed another group, with multiple trees from each group.

This is not a greenhouse per se but is my version of a"high tunnel " that other citrus growers use in cold-wintered areas of the southeastern USA. One closes doors on both ends of the tunnel during freezing weather and uses additional heat sources as needed, such as water barrels, to help modulate temperatures around the trees.

I understand what your point. I just want to tell you a narrow polytunnel is losing heat many times more then a square poly greenhouse, because the exposed surface area is bigger.I know you have the plants already in ground, but it would make sense to replant them in a square then rebuild the greenhouse. I would get much warmer and cheaper to heat it.

There's a wide range of variation among different tangerine and grapefruit varieties. There are many grapefruit varieties that are cold hardier than many different tangerine varieties. That's why it can be misleading to say that mandarins are more cold hardy than grapefruit. Historically a lot of these tangerine varieties got bred in North Africa, so apparently they lost their cold hardiness genes.

Citrumelo (has survived in zone 7b in the South, and a few cases growing in zone 8 England)Ichang papeda (cold hardy down to the border of zone 8a/7b in the South, zone 8a in a sheltered location in the PNW)Yuzu 10 FChangsha 10-12 FArctic Frost 10-13 F or 10-15 F (this is a Changsha x Satsuma cross, a few people have suggested this may not really be much hardier than regular Satsuma)Ten Degree tangerine 11.5 F or 10-14 F (this is a Clementine x Yuzu cross)Keraji 12-14 F

-----"There are many grapefruit varieties that are cold hardier than many different tangerine varieties."-----

SoCal2warm, as you must actually believe the above quote, then please name them so others can evaluate your statement.

Duncan and Marsh (both white grapefruits) are commonly regarded as the most cold hardy grapefruit varieties.

There are a few seedling grapefruits that purportedly have survived in South Carolina (though anecdotal evidence suggests they sometimes die back during very cold years but in some cases can regrow).There is even one instance of a white grapefruit originally grown from seed which managed to survive in very sheltered spot in London (yet outside uncovered).(see thread here: http://tropicalfruitforum.com/index.php?topic=26155.0 )

I just meant that, in general, mandarins are more cold hardy than grapefruits, but some grapefruits, like Duncan, have more cold hardiness than some mandarin varieties like Clementine, so there can be considerable overlap depending on the exact variety. (Of course Duncan is not as hardy as Satsuma mandarin)

The hardiest variety of the common mandarin varieties is Satsuma. ( "common" meaning commercially edible quality)Although it would be a bit more accurate to say that Satsuma is actually really more of a subgroup of mandarins, with several different varieties within it.

FF-6-15-150 is a new mandarin variety that came out of a USDA breeding program in Florida which is rated to have as much cold hardiness as Satsuma mandarin (though not related to Satsuma). It has Orlando tangelo in its parentage.Lee mandarin x Orlando Tangelo Lee mandarin itself was bred from Clementine x Orlando TangeloOrlando tangelo is a slightly cold hardier sibling of Minneola Tangelo, being bred from a cross between Dancy mandarin x Duncan grapefruitFF-6-15-150 resembles a small orange or Page mandarin, said to have good flavor good for juicing, though does contain a fair number of seeds.

Some more information on relative cold hardiness of different citrus, now going into some of the varieties that are not very hardy

Citron is basically the ancestral species from which lemons descend from. (ordinary lemons)Citron has even less cold hardiness than common lemons. Common lemon varieties in turn do not have as much hardiness as Meyer lemon, which is believed to have originally resulted from hybridization between mandarin and citron in China, and has a separate (but somewhat similar) ancestry from other lemons whose history dates to Southern Europe.

Limes have less hardiness than lemons, making them the least hardy common citrus group. Limes are mainly descended from an the citrus species C. micrantha, which has even less cold hardiness than limes. One exception, Kaffir lime is in a separate group, believed to be descended from an origin citrus species known as Khasi papeda (C. latipes), and has some small degree of cold hardiness, more than lemons.

In form, the fruits of citron appear quite similar to Ichang papeda, in fact the two species may be less distantly related to each other than to other citrus species, but whereas Ichang papeda is the second most cold hardy citrus species, citron is the second least cold hardy species. (so quite a difference in terms of level of cold hardiness!)

I've been wanting to attempt a hybrid between citron and Ichang papeda as the first step in trying to breed a lemon (a true lemon) with more cold hardiness. Maybe the 2nd step would be to cross the result with Duncan grapefruit or pomelo, or something like that.

While Meyer lemon is the cold hardiest of the true lemon varieties, and I am a really big fan of lemons, I'm just not really a big fan of Meyer lemons.(But that's personal preference, some people say Meyer is their favorite lemon variety)